1. Field of the Invention
The present invention relates to a NTC thermistor composition based on barium titanate, which is applied to an inrush current preventing element for a switching regulator or the like.
2. Description of the Background Art
In order to prevent an inrush current upon power supply to a switching regulator or the like, employed is a negative temperature coefficient thermistor whose composition comprises spinel-type transition metal oxide whose resistance is reduced with an increase in temperature. This thermistor suppress the inrush current supplied to the switching regulator or the like due to a high resistance under the room temperature, while its electric power consumption is reduced in a stationary state since the resistance is thereafter reduced by temperature rising caused by self-heating. Thus, such a negative temperature coefficient thermistor is widely employed as an inrush current preventing element.
However, a conventional negative temperature coefficient thermistor which is applied to a switching regulator or the like has a B constant, expressing the relationship between temperature and resistance by the equation R(T)=R.sub.0 exp(B/T) at 2000 to 4000K. Therefore, the resistivity of conventional negative temperature coefficient thermistor is so readily influenced by a change of temperature that its initial resistance is extremely varied with changes in atmospheric temperature to cause a variation in the rise characteristic. In particular, at a low temperature of less than 0.degree. C., an inrush current preventing element with this thermistor takes much time to achieve a stationary state due to self-heating caused by high resistance.
In order to solve the aforementioned problem, required is a negative temperature coefficient thermistor whose B constant is small around ordinary temperatures (-50.degree. to 50.degree. C.) and increases at higher temperatures beyond the said range.
In relation to such a negative temperature coefficient thermistor, there has been proposed an element which is prepared by adding 20 percent by weight of Li.sub.2 CO.sub.3 to barium titanate (BaTiO.sub.3), as disclosed in Japanese Patent Publication No. 48-6352 (1973).
In this element, however, power consumption is disadvantageously increased in a stationary state due to high specific resistance of at least 10.sup.-5 .OMEGA..multidot.cm at a temperature of 140.degree. C., although its B constant is abruptly increased when the temperature exceeds the phase transition point.